Fischer-Tropsch processes for hydrocarbon synthesis from CO and H2 (syngas) are known to produce gaseous and liquid hydrocarbons as well as oxygenates which, in general, follow the well-known Anderson-Schulz-Flory product distribution.
These reactions can be carried out in fixed, fluidised or slurry bed reactors. The production of olefins and liquid fuels, especially in the gasoline range products, is most favoured by synthesis carried out in a two-phase fluidised bed reactor operating at 350° C. and 20 bar or higher pressures and usually utilising a fused alkali promoted iron catalyst. This is known as a high temperature Fischer-Tropsch process.
In terms of the ideal Anderson-Schulz-Flory product distribution it is clear that the gasoline (C5 to C11) and diesel (C12 to C18) selectivities are limited to values of about 48% and 25% respectively, while combined liquid fuels selectivity has a maximum value of around 65%. In a high temperature Fischer-Tropsch process performed in a fluidised bed reactor, the optimum liquid fuel yield might not be realised, thus resulting in a lower liquid fuel selectivity. In addition to this relatively low liquid fuel yield, the Fischer-Tropsch process has a further disadvantage in that the product spectrum mainly consists of linear hydrocarbons. This is a disadvantage with respect to gasoline quality, since linear molecules have a very low octane number. Fischer-Tropsch gasoline thus requires either further work-up to convert the product to one with a higher octane number, or the addition of high-octane compounds to the gasoline pool.
It is known that the Fischer-Tropsch product spectrum can be worked-up to high octane gasoline range fuels by using an acidic catalyst such as a zeolite catalyst. Such a work-up has the disadvantage that it adds to the production costs of the liquid fuel.
In order to reduce the above disadvantage it has been attempted to combine a Fischer-Tropsch catalyst with a zeolite catalyst in order to prepare high octane gasoline range fuels directly from CO and H2. In such a system the idea is that the Fischer-Tropsch catalyst should catalyse the conversion of CO and H2 to hydrocarbons, and the acid catalyst should convert the resulting olefinic and oxygenated hydrocarbons to gasoline range products that are highly branched and high in aromatics.
U.S. Pat. Nos. 4,086,262, 4,279,830, 4,361,503, 4,269,783, 4,172,843, 4,463,101, 4,298,695, 4,304,871, 4,556,645, 4,652,538 all disclose the combined use of a hydrocarbon synthesis catalyst and an acidic catalyst in the preparation of hydrocarbons from syngas. The two catalytic functions have been combined in a variety of ways, ranging from a single reactor containing both catalytic functions to a dual reactor arrangement with the two catalytic functions in subsequent reactors. Different reaction conditions and different catalysts are disclosed in these patents.
It is recognised (eg. U.S. Pat. No. 4,298,695) that the addition of alkali promoters to the iron based Fischer-Tropsch catalyst in a bi-functional process (Fischer-Tropsch catalyst and acidic catalyst) is undesirable, since these promoters tend to migrate to the acidic catalyst with a resultant poisoning of the acid sites.
However, it is known that iron based Fischer-Tropsch catalysts with a low alkaline promoter level tend to produce light hydrocarbons that are not desirable for gasoline production since they do not fall in the gasoline range of C5 to C11 and are also not easily converted to this range.
Therefore, should a low level of alkali promoter be considered in the production of liquid fuel from syngas, the reaction conditions would be selected to produce a heavier hydrocarbon product despite the low alkali level. It is well-known that a high H2:CO ratio in the feed favours products which are not desirable for gasoline production. Accordingly if a low level of alkali promoter is considered a relatively low H2:CO ratio in the feed will be considered to avoid production of excessively light hydrocarbons.
Surprisingly it has now been found that if hydrocarbon synthesis of syngas is carried out in the presence of a hydrocarbon synthesis catalyst and an acid catalyst under conditions where:                i) the hydrocarbon synthesis catalyst includes a low level of alkali metal; and        ii) the hydrogen to carbon monoxide ratio of the syngas feed stream is relatively high,a hydrocarbon product is produced that is suitable for use as gasoline. This is true even if the synthesis is carried out under high temperature Fischer-Tropsch conditions.        